Kinetics of Dissolution and Recrystallization of Sodium Chloride at Controlled Relative Humidity

Both producers and users of divided solids regularly face the problem of caking after periods of storage and/or transport. Particle agglomeration depends not only on powder water content, temperature and applied pressure, but also on the interactions between the solid substance and water molecules present in the atmosphere, i.e. on relative humidity (RH) at which the product is stored. Ambient humidity plays an important role in most events leading to caking: capillary condensation of water at contact points between particles, subsequent dissolution of a solid and formation of a saturated solution eventually followed by precipitation of the solid during the evaporation of water. Here, we focus on the kinetics of dissolution followed by evapo-recrystallization of a hygroscopic sodium chloride powder under controlled temperature and RH, with the aim of anticipating caking by predicting rates of water uptake and loss under industrial conditions. Precise measurements of water uptake show that the rate of dissolution is proportional to the difference between the imposed RH and deliquescence RH, and follows a model based on the kinetic theory of gases. Evaporation seems to be governed by more complex phenomena related to the mechanism of crystal growth from a supersaturated salt solution.